Oxymethylene polymers having recurring --CH.sub.2 O-- units have been known for many years. They may be prepared, for example, by the polymerization of anhydrous formaldehyde or by the polymerization of trioxane, which is a cyclic trimer of formaldehyde.
The usefulness of oxymethylene polymers in molding compositions has also been known for many years. However, molding compositions comprising polyoxymethylene are, unfortunately, well known for their propensity to discolor over time either at ambient temperatures or at elevated temperatures. And, as is also well known, color retention of polyoxymethylene molding compositions is a necessary criteria when polyoxymethylene compositions are employed in a vast majority of applications (e.g., as automotive components).
Over the years, many attempts have been made to discover additives for polyoxymethylene molding compositions which effectively suppress color reactions (i.e., discoloration). For example, U.S. Pat. No. 3,204,014 to Green (the entire content of which is expressly incorporated herein by reference) discloses that polyoxymethylene compositions comprising vinylpyrrolidone, a phenolic antioxidant, and a dialkyl ester of 3,3'-thiodipropionic acid exhibit good color retention.
In addition, it is known from U.S. Pat. No. 3,488,303 to Heinz (the entire content of which is expressly incorporated herein by reference) that the problem of discoloration can be alleviated to a certain extent by the incorporation of a lanthanide salt in a polyoxymethylene composition.
While the above-noted polyoxymethylene formulations do provide some level of color stability, additional improvements in this regard are still needed. This is especially true with the more recent developments in thermally stable polyoxymethylene compositions. These new compositions, which may comprise additional stabilizer ingredients, provide further color reaction complications due to their additional ingredients. See, for example, U.S. Pat. No. 4,727,106 to Paul et al, the entire content of which is expressly incorporated herein by reference.
Further, and in addition to the prevention of discoloration, any commercially successful polyoxymethylene composition must also exhibit desirable processability characteristics. That is, during molding of parts, the composition must be capable of releasing easily from the mold without forming significant amounts of mold deposits. Otherwise, the molded part itself may not be useable due to improper and/or inconsistent part surfaces or unacceptably high surface gloss levels (particularly where parts having low surface gloss levels are desired).
Thus, it is towards achieving improved color stabilized polyoxymethylene compositions, via incorporation of an easily processable stabilizer additive, that the present invention is directed.